The invention of observation apparatus designed to observe an observation object on an observation plane at a desired position in an optical-axis direction of an objective lens is disclosed in Patent Literatures 1 and 2. The observation apparatus disclosed in these Literatures is configured to detect a front face or a back face of slide glass as a reference observation plane by making use of the large intensity of reflection from the front face or the back face of the slide glass with the observation object thereon, to move the slide glass in the optical-axis direction by a distance between a desired observation plane in the observation object and the reference observation plane, and then to observe the observation object on the desired observation plane.
For example, when the observation object is a cell, an observer sometimes desires to acquire physiological information of the observation object and sometimes desires to acquire morphological information of the observation object. For acquiring the physiological information of the observation object, a specific part in the observation object is labeled with a fluorochrome, the observation object is irradiated with excitation light, and an image of fluorescence generated upon the irradiation is taken. On the other hand, for acquiring the morphological information of the observation object, low-coherence light is split into two beams as a first branch beam and a second branch beam, interference is caused between first reflection made upon application of the first branch beam onto the observation object and second reflection made upon application of the second branch beam onto a reference mirror, and a resultant interference image between them is taken (cf. Non Patent Literatures 1 and 2).